Hydrogen Lyman Alpha Spectral Line Profiles in Coronal Holes from 1.5 - 6.5 Solar Radii

Astronomy and Astrophysics – Astrophysics

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7511 Coronal Holes, 7549 Ultraviolet Emissions

Scientific paper

UVCS has made detailed measurements of H I Lyα spectral line profiles in a polar coronal hole at projected heliocentric heights from 3.5 to 6.5 R\odot during 1998 January 5 -- 11. Similar polar coronal hole measurements were made during 1998 June 16 -- 21. Earlier UVCS observations obtained at 1.5 to 2.5 R\odot are used for comparison. In addition, new measurements are being made for the current phase of the solar cycle. From these measurements we derive 1/e half widths of coronal velocity distributions at the observed heights. The velocity distribution includes all motions contributing to the Doppler shifts along the line of sight (LOS). We also measure absolute intensities that can be used to derive outflow speeds via a Doppler dimming analysis. At large heights in coronal holes, the outflowing coronal plasma becomes nearly collisionless and the ionization balance is believed to become frozen. H I Lyα profile measurements characterize the neutral hydrogen velocity distribution, which at lower heights can also be used to describe the proton distribution. However, in the regions above 3~R\odot, the H0 velocity distribution may not be the same as that for the protons because the characteristic time for charge transfer between H0 and protons becomes longer than the time it takes for the plasma to flow through a density scale height. Hence, the H0 velocity distribution may not be directly affected by transverse wave motion or wave damping. An indication of an adiabatic radial decrease in the neutral hydrogen temperature would indicate a decoupling of the protons and neutral hydrogen, and also indicate the absence of mechanisms that would heat the neutrals. We will compare the observations with the predictions of a theoretical model of the combined electron, proton and neutral hydrogen plasma. Such a comparison could lead to an improved knowledge of the proton heating at these large heights. This work is supported by the National Aeronautics and Space Administration (NASA) under Grant NNX07AL72G to the Smithsonian Astrophysical Observatory.

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